Demand on cellular network capacity is growing very fast with the growth of smart phones and the wide usage of video applications. There are projections that the data traffic on the cellular wireless system will grow by 26× in 5 years. At the same time the revenue per bit ($/MByte) for service providers is going down. With spectrum being limited and expensive, capacity has to be increased by squeezing more bits into the same bandwidth. Long term Evolution (LTE) standard from the 3GPP standard body has improved the amount of data that can be packed in a given bandwidth (bits/Hz), but the improvement is incremental. The standards body 3GPP has charted out a few schemes to improve the capacity of the system, such as, Beamforming, MIMO, MU-MIMO (or network MIMO) coordinated multi-point systems (CoMP) (see 3GPP standards documents, including 3GPP TS 36.211: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”). The recent book “LTE—The UMTS Long Term Evolution from theory to practice” by S. Sesia et al (Wiley 2011), gives a good background on the subject. The other approach to improve capacity is to use small cells (Pico cells, Femto cells). By more frequency reuse capacity can potentially be improved. However, apart from increasing infrastructure costs, small cells exacerbate interference between cells, and cell edge coverage becomes a bottleneck. All these techniques have practical limitations and these have to be fixed before any large capacity growth is possible.
The transmit channel is the channel for the signal path from the transmitter to the receiver. The transmit channel can be estimated at the receiver. However, the transmit channel is not known at the transmitter. If the transmit channel information is available at the transmitter then it can optimize the signal transmission to match its channel. In many cases the receiver station may not be able to send back the channel information due to constraints on bandwidth.